Changes in the ratio of cis-trans to trans-trans abscisic acid during ripening of apple fruits.

Immediately after harvest, abscisic acid (ABA) extracted from fruits of the apple cultivar 'Golden Delicious' comprised solely the cis-trans isomer. During postharvest ripening, however, trans-trans ABA accumulated and finally exceeded the level of cis-trans ABA. The two geometrical isomers were separated and identified by high-performance liquid chromatography (HPLC) and combined gas chromatography-mass spectrometry. After purification by HPLC the putative trans-trans isomer yielded considerable quantities of cis-trans ABA, when irradiated with UV light. This isomerization was more rapid than the reverse reaction. The physiological significance of the accumulation of trans-trans ABA is discussed, as well as the applications of these results in the use of trans-trans ABA as an internal standard during the extraction and quantification of ABA from plant tissues.